In the assembling of a gas turbine engine several interconnecting components are assembled together, all of which are machined with high precision although within a specified dimensional tolerance. The cumulative effect of these tolerances in assemblies of multiple parts must be accommodated especially when extremely high precision in assembly is required.
One example of such precision is in the assembly of high-pressure turbines and their associated turbine shrouds immediately downstream of a combuster in the gas turbine engine. It has been estimated that the tip clearance, between the tips of turbine blades and the turbine shroud surrounding the rotating blades, is so critical to engine performance that each 0.001-inch of excess tip gap results in approximately a 0.25% decrease in engine performance. As a result, extreme care is taken in ensuring that high-pressure turbine blades are accurately assembled together with their associated turbine shrouds.
In the prior art the method of assembling high-pressure turbines and their shrouds involves assembling the engine including the combuster and high-pressure turbine shaft within acceptable assembly tolerances. The high-pressure turbines are then mounted to the high-pressure turbine shaft and invariably the rotational axis of the turbine is somewhat eccentric of the longitudinal axis of the combuster due to accumulation of machining tolerances.
In order to accurately fit the turbine shroud about the blades of the turbine with minimal tip gap, it is common practice to custom grind the internal surface of each shroud to precisely match the eccentricity and outside diameter of the turbine blades in operation.
As will be appreciated, the withdrawal of the turbine shroud from assembly operations and precision grinding involve significant delay in the assembly operation as well as high costs as a result of the skilled labour involved in the process. In effect, while the custom grinding operation is being carried out on the turbine shrouds, the assembly operation is halted imposing significant manufacturing difficulties due to space and scheduling commitments.
In general, all machinery assembly operations can benefit from the use of standardized components rather than custom fitting each component as required. Standardization of components and simplification of the assembly process, inevitably will reduce costs and increase the speed of production.
It is an object of the present invention, therefore, to eliminate the custom grinding of turbine shrouds and permit the standardization of turbine shroud manufacture regardless of the eccentricity produced in assembly of turbines used in gas turbine engines.
It is a further object of the invention to provide a simple means by which eccentricity of turbines can be accommodated without custom grinding or removal of components from the flow of assembly and production.
It is a further object of the invention to enable eccentricity of the assembled turbines to be accommodated without significant alteration to the prior art structure of gas turbine engine components. Ideally, only minimal modification is desirable.